The cathode-ray tube is the most expensive component used in a color television receiver and damage to it may result in a major repair bill amounting to 25% or more of the initial cost of the receiver. The second most expensive component is the flyback transformer.
For picture tube protection most receivers employ a beam current limiting mechanism which senses the average picture tube beam current. When a pre-determined average beam current is reached circuits are activated which limit the level of the input signal to the video drive amplifier stages and hence the average beam current. However, the protection supplied by such circuits does not extend to a failure occurring in the video amplifier stages located down-stream in the signal path from the point at which the beam current limiting signal is applied. If the failure causes an increase in the cathode drive the resulting increase in beam current may damage or shorten the life of the picture tube. Such damage may occur before the primary overload protection circuits or devices are activated. Such circuits and devices are usually comprised of fuses, thermal cut-outs and crowbar circuits which are designed to protect against fire hazards resulting from catastrophic component failures. Fuses and other similar protection systems used in the receiver cannot be set to trip reliably with relatively small increased increments of power consumption. Attempts to design a fuse-type system to protect against occasional power surges would result in annoying and costly service calls. In general, attaining accurate and reliable protection using overload-sensing circuits is difficult because of component and production tolerances.
One of the requirements for a high performance television receiver is the maintenance of a small focussed spot during the electron beam scan of peak white areas of the picture. The peak white areas of the picture require high beam currents. While many problems are encountered in the attainment of the small focussed spot, one of the most fundamental problems is that of providing the power demands of the picture tube during the white highlights consistent with allowable limits for dissipation and x-radiation. In general, the solution to the problem requires the use of low impedance power supplies to deliver the necessary power during periods of high beam current to aid in maintaining essentially constant operating voltages. The use of a low impedance high voltage power supply for the electron beam current increases the probability of damage to the picture tube and supply during fault conditions.